Story highlights

Last season's Champions League finalists Barcelona lose for first time in 2012-13

Spanish side could have qualified for last 16 but went down 2-1 away to Celtic

Champions Chelsea snatch injury-time win at home to Shakhtar Donetsk

Manchester United through to knockout stage after fourth successive victory

Barcelona's unbeaten run under new coach Tito Vilanova came to an unexpected halt on Wednesday as the Spanish side failed to clinch a place in the knockout stage of the European Champions League after losing 2-1 at Celtic.

Lionel Messi's late goal, his 74th of 2012, proved a mere consolation as the Scottish champions celebrated a famous victory to mark 125 years of existence.

The result left Barca still on top of Group G, while Celtic's hopes of reaching the last 16 were greatly improved as a first-half goal from Victor Wanyama and an 83rd-minute strike from teenage substitute Tony Watt put Neil Lennon's team in second place with two games left.

Spartak Moscow dropped to the bottom of the group after losing 2-0 at Benfica, with Oscar Cardozo scoring both second-half goals for the Portuguese team.

The Paraguay striker then missed a penalty, hitting the crossbar after Argentine defender Nicolas Pareja was sent off.

Barcelona traveled to Parkhead as the only team from Europe's top leagues still unbeaten this season following weekend defeats for Juventus and Paris Saint-Germain, but went behind in the 21st minute when Kenyan midfielder Wanyama headed in Charlie Mulgrew's corner.

Managerial maestros – Jose Mourinho gestures during the El Clasico clash with his Barcelona counterpart Tito Vilanova in the background.

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Photos:Spanish heavyweights battle El Clasico draw

Political protests – Barcelona's decision to invite a former captured Israeli soldier Gilad Shalit to the El Clasico clash was met with protests in Palestine.

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Photos:Austerity gives El Clasico new twist

Photos:Austerity gives El Clasico new twist

Catalan colors – Barcelona's Nou Camp stadium will be transformed into a giant Catalan flag prior to kick off in Sunday's "El Clasico" match against Real Madrid. A total of 98,000 placards will proudly display bands of red and gold.

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Photos:Austerity gives El Clasico new twist

Messi vs. Ronaldo – "El Clasico" also brings together the best two players on the planet. Cristiano Ronaldo of Real Madrid, right, has started the season in fine form and scored a hat-trick in Wednesday's European Champions League defeat of Ajax. But it is Barca's Lionel Messi who is revered by many as the finest player in the world. The Argentine has won the FIFA World Player of the Year award in each of the last three seasons.

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Photos:Austerity gives El Clasico new twist

Mourinho meets Tito – Real coach Jose Mourinho had a fiery confrontation with then Barcelona assistant coach Tito Vilanova in August 2011. Mourinho poked Vilanova in the eye amid a scramble on the touchline of last year's Super Cup encounter. Vilanova has since gone on to replace Josep Guardiola as Barca coach.

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Photos:Austerity gives El Clasico new twist

Bitterness and bile – The rivalry between Real Madrid and Barca is one of the most intense in football, and players who move between the two clubs are rarely forgiven. When Portugal's Luis Figo returned to the Nou Camp after moving to Real in 2000, fans threw numerous objects in his direction -- including the pig's head pictured here.

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Photos:Austerity gives El Clasico new twist

Calls for independence – The president of semi-autonomous Catalonia is Artur Mas.He has been accused of using the debt crisis engulfing Spain as a way of garnering more support for the independence movement.

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Photos:Austerity gives El Clasico new twist

September 11, 1714 – The Catalan National Day is September 11, commemorating the day in 1714 when the Spain monarchy defeated Catalan troops in the War of the Spain Succession. The day is used to showcase the region's culture and traditions, as well as to campaign for secession from Spain.

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Photos:Austerity gives El Clasico new twist

Pep rally – Josep Guardiola, right, gave his support to Europe's victorious Ryder Cup campaign in Illinois last weekend. The former Barca coach has also echoed calls for Catalan secession, declaring on the region's national day: "Here's one more vote for independence."

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Photos:Austerity gives El Clasico new twist

A history of conflict – Catalan customs and the region's language were aggressively repressed by general Franco during the Spanish Civil War, which was fought between 1936 and 1939.

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Photos:Austerity gives El Clasico new twist

Austerity measures – The Spanish financial crisis has led to a raft of anti-austerity measures. Protests against cuts have been seen on the streets of Madrid, where a minority of protesters have been involved in violent clashes with the police.

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The 18-year-old Watt, making his first appearance in Europe's top club competition, made it 2-0 after veteran midfielder Xavi failed to cut out a long clearance by Celtic keeper Fraser Forster.

Messi netted at the start of injury time after an initial block by Forster but Barca could not repeat the last-gasp heroics of Celtic's visit to the Nou Camp last month, when Vilanova's team snatched a 2-1 victory.

It was one of the biggest results in Celtic's history, behind winning the European Cup in 1967 and losing in the final three years later.

"It's right up there, one of the proudest nights in my career," Lennon told reporters. "It was a monumental effort from all those players and on the anniversary of club, it's a very special occasion.

"I wanted them to do themselves justice and they've even surpassed that, they've beaten the best club in the world.

"You try to weather the storm, you know it's coming and there's not a lot you can do. Then young Tony comes on, gives us new lease of life and scores a great second goal, and he'll go down in history now."

Another unbeaten run came to an end as Barcelona's conquerors in last season's final, Chelsea, boosted hopes of reaching the last 16 after stealing a 3-2 win at home to Shakhtar Donetsk in the fourth minute of injury time.

Substitute Victor Moses headed in Juan Mata's corner to avenge last month's 2-1 defeat away to the Ukrainian league leaders, who paid for defensive lapses at Stamford Bridge.

The English team took the lead in the sixth minute when Spain striker Fernando Torres charged down an attempted clearance by goalkeeper Andriy Pyatov following a poor backpass by Yaroslav Rakitskiy.

Longtime Chelsea target Willian leveled in the ninth minute but fellow Brazilian midfielder Oscar gave the home team the halftime advantage with a superb long-range volley after Pyatov - racing out of his area -- headed a clearance straight into his path.

Willian equalized again straight after the break with an almost carbon copy of his first goal, this time with his left foot, but Chelsea hit back again as John Obi Mikel had a goal ruled out for offside and then 21-year-old Nigeria forward Moses followed up his goal in Saturday's Premier League draw with Swansea.

It left Shakhtar on top of Group E on goal difference from Chelsea, with Juventus a point adrift in third following a 4-0 win at home to Nordsjaelland.

The Italian champions ended a run of nine successive draws in European competition as Claudio Marchisio, Arturo Vidal and Sebastian Giovinco scored in the first half, while second-half substitute Fabio Quagliarella headed the other against the Danish league winners.

Manchester United ensured there would be no repeat of last season's embarrassment by booking a place in the last 16 with two matches to spare after a 3-1 win at Portuguese club Braga.

United failed to qualify from a group including Benfica, Basel and Otelul Galati as manager Alex Ferguson paid the price for resting his big names, but this time there has been no mistake with four wins from four in Group H.

Braga led through Alan's early penalty, but substitute Robin van Persie leveled before Wayne Rooney scored a penalty and a third forward, Javier Hernandez, bundled in another as United matched the late comeback in the 3-2 win at Old Trafford two weeks ago.

Braga dropped to the bottom of the group after Galatasaray joined Cluj on four points with a 3-1 win in Romania.

Turkey striker Burak Yilmaz scored a hat-trick to earn his club's first win of the European campaign.

Bayern Munich moved top of Group H with a 6-1 hammering of Lille that ended the French club's hopes of qualification.